When a horse suffers a skeletal muscle injury the normal healing process results in scar tissue formation at the injury site. Scar tissue is relatively inelastic and, depending on how much forms and where it develops, it can compromise motion by reducing the affected muscle's elasticity. In horses, this movement restriction in hamstring muscles is called fibrotic myopathy. Typically, only one hind limb is affected, but both limbs can be affected—in which case one side is more severely affected than the other. While this condition has long been diagnosed, veterinarians still don't fully understand its impact on locomotion, and current treatment options aren't always effective.

To that end, researchers at the University of Missouri (UM) carried out a multifaceted study to evaluate fibrotic myopathy's effects on equine locomotion and laser surgery's ability to cut through scar tissue in hamstring muscles. The team also examined whether a rapid return to exercise encourages affected horses' tissues to remain more elastic.

“Exercise soon after surgery helps to stretch the immature scar tissue, leading to longer (i.e., less restrictive) mature fibrotic tissue, which will not produce significant effects on the forward advancement of the leg,” said study co-author Marco Lopes, MV, MS, PhD, research associate at the MU College of Veterinary Medicine.

Lopes and colleagues performed gait analysis with high-speed cameras on eight horses with fibrotic myopathy before the horses underwent laser surgery to cut the fibrotic masses. The team encouraged owners to begin hand walking the horses within a day of surgery. They carried out gait analysis again one to two days and six to 11 weeks postoperatively.

Upon reviewing their results, the team found that before surgery, all the horses had some sort of a hitch in their step. Scientists noticed that the affected leg's forward swing during movement was abruptly curtailed by a sudden, awkward downward and back-sliding step.

After surgery, the researchers reported that movement had improved in six of the eight horses. The previously seen abrupt downward and backward motion of the foot before impact was reduced, along with some of the hoof height disparities between the affected and normal leg.

Due to the surgical technique and use of the laser, the horses' postsurgical healing process was relatively uncomplicated, the team noted.

Four to five years after surgery, Lopes and colleagues followed up with six of the eight owners. Four of them could no longer see an abnormality in their horse’s gait, one saw improvement, and one saw no change.

Because scar tissue isn’t painful for horses, some researchers question the need to treat it. Scar tissue also forms differently from horse to horse, so surgery does not always effectively restore normal movement, as was the case with one horse in the study.

“The amount and shape of the fibrotic tissue that forms after the original injury are variable,” said Lopes. “We cannot say for sure but it is possible that not enough fibrotic tissue was transected (i.e., a major band of fibrotic tissue was left intact) in one horse leading to minimal or no improvement shortly after surgery.”

Lopes also said this is the first time that gait analysis was used to assess gait changes associated with fibrotic myopathy before and after surgery, and the technology gave scientists a clear, objective idea of what was going on—a big change from the days when physicians had to diagnose the issue solely by sight.

“Our vision cannot detect fast motion, and our brain cannot keep track of all details that our eyes can see,” explained Lopes. “When gait analysis is performed with the tools we used, we study all details of the gait and we can accurately quantify things (i.e., how high, how far forward, and how fast is the leg moving). Thus, in this study we were able to accurately measure features of the hind limb motion that are virtually invisible even to the most experienced veterinarians.

“Unfortunately the gait abnormalities associated with fibrotic myopathy were not permanently eliminated in all horses, and we found evidence of reformation of restrictive fibrotic tissue in some horses," Lopes concluded. "It makes sense to try additional measures with the aim of minimizing reformation of restrictive fibrotic tissue.”

About the Author

Natalie Voss is a freelance writer and editor based in Kentucky. She received her bachelor's degree in equine science from the University of Kentucky and has worked in public relations for equine businesses and organizations. She spends her spare time riding her Draft cross, Jitterbug.

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